[PATCH] sch_red: fix weighted average calculation

This patch fixes an apparent bug in the running weighted average calculation
used in the RED algorithm.

Going by the described formula:
	   qavg = qavg*(1-W) + backlog*W
	=> qavg = qavg + (backlog - qavg) * W

... with W converted to a pre-calculated shift, this then becomes:
	qavg = qavg + (backlog - qavg) >> logW

... giving the modified expression introduced by this patch.

Signed-off-by: John Dowdal <redacted>
---
 include/net/red.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/net/red.h b/include/net/red.h
index ef46058..05960a4 100644
--- a/include/net/red.h
+++ b/include/net/red.h

@@ -287,7 +287,7 @@ static inline unsigned long red_calc_qavg_no_idle_time(const struct red_parms *p
 	 *
 	 * --ANK (980924)
 	 */
-	return v->qavg + (backlog - (v->qavg >> p->Wlog));
+	return v->qavg + (backlog - v->qavg) >> p->Wlog;
 }
 
 static inline unsigned long red_calc_qavg(const struct red_parms *p,

-- 
1.7.9.5

On Thu, 2012-09-13 at 09:43 -0400, Cyril Chemparathy wrote:

This is going to be a FPP (Frequently Posted Patch)

Current formulae is fine.

Thats because backlog, at start of red_calc_qavg_no_idle_time() is not
yet scaled by p->Wlog. v->avg is scaled, but not backlog.

Have you tested RED after your patch ?

Eric, thank you for reviewing the code.  I now see the problem with the patch since backlog is an integer and qavg is a fixed point number at logW.  

We are considering another patch to update the comments to this code (with the actual C code change reverted) to stop the FPP by showing the derivation of the equation in the comments.  Does this sound good?


-----Original Message-----
From: Eric Dumazet [mailto:eric.dumazet@gmail.com] 
Sent: Thursday, September 13, 2012 9:54 AM
To: Chemparathy, Cyril
Cc: linux-kernel@vger.kernel.org; netdev@vger.kernel.org; davem@davemloft.net; david.ward@ll.mit.edu; Dowdal, John; paul.gortmaker@windriver.com
Subject: Re: [PATCH] sch_red: fix weighted average calculation

On Thu, 2012-09-13 at 09:43 -0400, Cyril Chemparathy wrote:

This is going to be a FPP (Frequently Posted Patch)

Current formulae is fine.

Thats because backlog, at start of red_calc_qavg_no_idle_time() is not
yet scaled by p->Wlog. v->avg is scaled, but not backlog.

Have you tested RED after your patch ?

On Fri, 2012-09-14 at 13:01 +0000, Dowdal, John wrote:

Eric, thank you for reviewing the code.  I now see the problem with
the patch since backlog is an integer and qavg is a fixed point number
at logW.  

We are considering another patch to update the comments to this code
(with the actual C code change reverted) to stop the FPP by showing
the derivation of the equation in the comments.  Does this sound good?

This would be good, please do so.

Thanks